It is standard practice to use large storage tanks also known as vessels (hereafter “tanks”) for storing a variety of products including liquids, such as beverage and petroleum products, as well as solid products such as powders. Conventionally, such storage tanks are usually made from non-stainless steel plates, and in the case of petroleum products the tanks are generally made from ¼ inch (0.63 cm) to ½ inch (1.27 cm) thick steel plates welded together. The dimensions of conventional storage tanks usually range in the hundreds of feet (100 feet=30.5 meters) in height and hundreds of feet in diameter.
Contact electromagnetic detection and sensing such as based on GWR may be used to determine the presence or signatures (object classification or shape) of objects, levels or the distance to the surface of materials, when other sensing methods may have difficulty in providing reliable and/or accurate information. For example, in the oil and gas industry, inaccurate or unreliable level measurements can incur a significant loss of profitability/revenue in tank level gauging applications. An error of 1 millimeter (mm) for a level measurement in storage tanks being 40 to 80 meters in diameter can correspond to a volumetric error of several cubic meters. Since the crude oil price is generally at least $70 per barrel (1 barrel=42 US gallons or 159 liters), the 1-mm error can result in thousands of dollars loss for one or more parties involved in trading and oil transfer. Measurements of a product level and/or interface between two products having a different dielectric constant located at distances greater than about 50 m below the top of the tank is also known to be a challenge for GWR level systems to provide accurate measurements.
In a known GWR system arrangement, the microwave components along the propagation path from the electronic block (including a processor, digital to analog converter (DAC) for transmitting and analog to digital converter (ADC) for receiving and a transceiver) which generates a microwave signal is launched along a guiding probe to the product surface/interface to be measured by including a 50 ohm coaxial cable coupled to the transceiver, with the coaxial cable being coupled to an overall essentially 50 ohms coaxial feed-through. The feed-through has its outer conductor “sleeve” either welded to the flange on a 120 to 180 ohm metal cylindrical tank nozzle (the impedance depending on the probe diameter and nozzle diameter) having an aperture, or is threaded to an aperture in the top surface of the tank over a tank aperture. The probe within the tank is electrically and mechanically connected to the central conductor of the feed-through device and its length is dependent on the application requirements. The impedance of the probe in free-space is about 370 ohms.